Copper interconnect systems which use conductive, metal-based cap layers

ABSTRACT

An integrated circuit (IC) may include a substrate, a first dielectric layer adjacent the substrate, and at least one trench in the first dielectric layer. The IC may also include a metal liner within the at least one trench, and a first conductive region including copper within the at least one trench. A cap layer including metal may be provided on the first conductive region. A second dielectric layer may be over the first conductive region and the cap layer. A dielectric etch stop and diffusion barrier layer may be over the second dielectric layer, and a via may be over the first conductive region and through the second dielectric layer and the cap layer. A diffusion barrier layer may be on sidewalls of the via, and an alloy seed layer including copper and at least one of tantalum, molybdenum, chromium, and tungsten may be over the diffusion barrier. The alloy seed layer may also be over the dielectric etch stop and diffusion barrier layer, and the alloy seed layer may be in contact with the first conductive region.

RELATED APPLICATION

The present application is a continuation of U.S. patent application No.11/278,914, filed Apr. 6, 2006, which is a continuation of U.S. patentapplication No. 10/803,475, filed Mar. 18, 2004, and issued Apr. 11,2006 as U.S. Pat. No. 7,026,714, which is based upon ProvisionalApplication No. 60/455,496, filed Mar. 18, 2003, the entire contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of electronics, and, moreparticularly, to semiconductor devices including copper interconnectsand related methods. Even more particularly, the invention relates toreliability improvements for copper interconnects.

BACKGROUND OF THE INVENTION

Doped seed layers have been disclosed in the prior art, their presencedirected to various objectives. These include U.S. Pat. No. 5,969,422 toTing et al.; U.S. Pat. No. 6,249,055 to Dubin; U.S. Pat. No. 6,181,012to Edelstein et al.; U.S. Pat. No. 6,066,892 to Ding et al.; and U.S.Pat. No. 6,461,675 to Paranjpe et al. But none of these inventionsdiscloses a copper interconnect system where a copper via to underlyingcopper interconnect makes a substantially direct copper to copperconnection without the presence of an intervening diffusion barrier. Thepresence of such a barrier tends to degrade the electromigrationresistance of the system.

Two prior art patents disclose where direct or near directcopper-to-copper interfaces are formed at the base of a copper via to anunderlying copper interconnect. U.S. Pat. No. 6,169,024 to Hussein formsa seed layer of varying thicknesses against a refractory metal diffusionbarrier as are common in the industry, and then etches both refractorymetal barrier and the dielectric cap material at the base of the viawith the seed material acting as a mask. Such a process is inherentlyextremely difficult to control, the main problem residing in controllingthe required wide variation in seed layer thicknesses.

U.S. Pat. No. 6,380,075 to Cabral, Jr. et al. discloses a process whichpurports a CVD method wherein the liner thickness is very thin ornonexistent at the base of the via and yet of sufficient thickness onthe horizontal surfaces under the copper interconnect regions. Such aprocess is also inherently extremely difficult to control. Neither ofthese prior art references discloses use of a metal-based cap layer,alloy seed layers, or methods of improved interface bonding.

U.S. Patent Application Publication No. 2002/0106895 A1 to Chungdiscloses a method to provide direct copper-to-copper contact at thebase of a copper dual-damascene via. But no seed layer is disclosed, andno methods for improving the adhesion at the various copper interfacesare disclosed. Upon application of high current density, any copperinterface where weak bonding of the adjoined film or material ispresent, tends to degrade the electromigration and stress inducedmigration failure rates.

U.S. Patent Application Publication No. 2003/0190829 A1 to Brennan alsoproposes a method to provide direct copper-to-copper contact at the baseof a copper dual-damascene via. But no capping or seed layers aredisclosed, and no methods for improving the adhesion of the variousinterfaces are disclosed. The method requires use of high dielectricconstant nitride-based etch stop and diffusion barriers. This tends todegrade RC delays in the copper interconnects. A more serious concern isthat the patent discloses no diffusion barriers for the edges of thecopper interconnects.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a copper interconnect system which uses aconductive, metal-based cap layer, and substantially directcopper-to-copper contact at the interface of the copper via to theunderlying copper interconnect. The invention, with its severalembodiments, is focused on damascene type implementations. Within thedual-damascene structure, doped, that is, alloy seed layers may be usedboth against dielectric (insulating) and metal-based or conductivediffusion barriers. The resulting structure has improved resistanceagainst electromigration (EM) and stress induced migration failures.

The invention provides enhanced adhesion or interfacial bonding for allcritical interfaces including: seed layer to dielectric diffusionbarrier layer; cap layer to main copper region; conductive trench linerbarrier materials to seed layer; and conductive trench liner barriermaterials to main surrounding inter-level dielectric (ILD). Use of highdielectric insulators may be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are schematic cross-sectional views of an integrated circuitduring manufacturing thereof in accordance with a first embodiment ofthe present invention.

FIGS. 4-6 are schematic cross-sectional views of an integrated circuitduring manufacturing thereof in accordance with a second embodiment ofthe present invention.

FIGS. 7 and 8 are schematic cross-sectional views of alternateembodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully with reference tothe accompanying drawings, in which preferred embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

The following first embodiment is directed to improving the variousweaknesses of prior art copper interconnect technology as discussed indetail above. These include weak bonding of the copper interconnect atthe various interfaces present. Among various improvements, thisinvention teaches the use of certain metals and alloys to improve thebonding. This results in greatly reduced interface diffusion rates. Thisimproves EM resistance, and reduces stress induced migration failurerates.

Referring to FIG. 1, a first Damascene-type interconnect comprisingcopper 3 is formed in an SiO₂ or low-k type insulator 1 that, in turn,is on or adjacent a substrate 30. A liner 4 is formed by PVD, CVD or by(atomic layer deposition) ALD or by other methods, of Ta, Ta/TaN,Ta/TaN/Ta or other combinations thereof, or, for adhesion enhancement,an alloy or mixture comprising Ti added to Ta, Cr, Mo, W, Rh, Ru orRe—such alloys yielding improved adhesion to copper based metal 3. Thebarriers may also contain nitrogen or Si. In general, nitrogen tends toreduce grain boundary diffusion rates, and small concentrations of Sitend to produce amorphous or more amorphous films. The liner 4 may havea thickness in the range of one monolayer to 200 Å. The added Ticoncentration would range from 1 to 30%. Diffusion barrier liners ofrhodium, ruthenium or rhenium may also be used as disclosed in U.S. Pat.No. 6,441,492, the entire contents of which are incorporated herein byreference.

Following the planarization step using CMP, as indicated by a planeshown by the dashed line 21, a selective metal-based cap 5 is then addedto the surface of interconnect 3 as described above. The cap 5 may beformed using electroless Co-based alloys, such as Co—W—P or Co—W—B, ormay be the Ru system described above, or another selectively depositedconductive system offering both a measure of good or improved adhesionto copper and an adequate diffusion barrier against penetration ofcopper into the surrounding interlevel dielectric ILD. (An alternativeembodiment for a Ru-based barrier is described below.)

Dielectric layer 2 is then formed over the capped interconnect.Dielectrics used for the ILD may be SiO₂-based or may be one of variouslow-k insulators as known in the art. These include films such as:oxides containing fluoride (FSG), SiLK™, Black Diamond™, various spin-onorganics, HSQ, various high-porosity SiO₂-based types, and many othersnow under investigation and refinement in the industry.

Etch stop and dielectric diffusion barriers 6 are then formed usingsilicon nitride, Si—C, Si—C—N—O, Si—O—N, or Si—C—N materials as areknown in the art. Dielectric layer 7 and etch stop and optionaldiffusion barrier layer 8 are added prior to the formation of opening 9.Opening 9 is then formed using photoresist layers, not shown, as isknown for the via for the first dual-damascene process. Differentialetch rates for the dielectric diffusion barriers are not required inthis process, so the materials may be selected for a minimum dielectricconstant. The Co-based barrier system 5 is then removed over the copper3 using argon bombardment or backsputter cleaning. For a Ru-basedembodiment, the Ru metal may be removed in fluorine-based RIE plasmachemistry. For a Ru-based cap an alternative is to remove the capmaterial upon etching the trench for the interconnect, that is, etch thefilm after the via first step. Photoresist is then added and a trenchfor interconnect 12 is formed by plasma etching as is known in the art.Fluorine or chlorine plasma chemistries do not appreciably attackexposed copper-based metal 3.

FIG. 1 shows the system at the intermediate step following opening ofthe trenches for a second overlying interconnect 12. FIG. 2 shows thestructure following the deposition and anisotropic etchback of aconductive diffusion barrier 8 which can be of similar composition tolayer 4. Alternately, the barrier may be a dielectric barrier such assilicon nitride, Si—C—N—O or Si—C—N. A diffusion barrier is depositedand is anisotropically etched back leaving edge protection layers 10(FIG. 2). Materials, such as those used for layer 8 could be used.Selective plasma etching chemistries may be employed using chlorine orfluorine or combinations thereof as are known in the art. The coppermaterial 3 is exposed at the base of the via. The exposed copper at thispoint may be cleaned and optionally passivated using displacementplating with a metal more noble than copper, such as Ag, Pd or Pt. Thisfilm thickness may be one monolayer up to about 50 Å. Following thisoptional plating, the surrounding dielectrics may be cleaned withchemical complexing agents. The exposed copper 3 may also be sputtercleaned prior to the formation of layer 11. The passivation stepprovides substantially no, or only partial, diffusion barrier propertiesat this copper 3 to copper 12 interface.

FIG. 3 shows the structure after deposition of an alloy seed layer 11.This alloy comprises copper and Ta, Cr, Mo, or W forming a mixture oralloy with improved adhesion to dielectric diffusion layer 6. Thedopants Ta and Cr are preferred. The alloy seed layer 11 may bedeposited by PVD, or other methods such as CVD, such that a reasonablyconformal film is created. The added refractory metals may be in atomicconcentrations ranging from 1 to 30% or more. Seed layer 11 is designedto be strongly copper-like and does not contain sufficient dopant oralloying material such that it behaves as a diffusion barrier. Coppermetal or copper alloy 12 may be then formed by electroplating or byelectroless plating as is known in the art. A cap 13 is added to thecopper or copper alloy layer 12 in like manner to layer 5. The copper orcopper-based metal 12 makes direct, or near direct contact, orequivalent direct contact, or contact without the intervening presenceof a substantial or diffusion-blocking thickness of an immiscible orpassive diffusion barrier, to copper or copper alloy metal 3. This mainregion copper 12 to lower level copper 3 contact is made through seedlayer 11 comprising copper.

Referring now additionally to FIGS. 4-6, a second embodiment provides aconductive coating or cap layer on the copper interconnect surface forthe purpose of interface diffusion rate reduction. The coating is not arobust diffusion barrier, and for that reason it is covered by adielectric diffusion barrier. The coating is etchable inhalide-containing anisotropic plasma etching chemistries (RIE), and maybe removed so that overlying copper vias may make a more directcopper-to-copper connection. This aids the prevention of copper fluxdivergence at the base of a via, a phenomenon which tends to reduce EMresistance. This embodiment also has no substantial diffusion barrierthickness remaining at the base of copper vias.

Following FIG. 4, a copper based interconnect 12 is formed oversubstrate 11 by a damascene-type process. The trench is lined withdiffusion barrier 13 as described in the first embodiment above. Thefilm 23 below the dashed line 22 is an insulating layer as is known inthe art, and is described in the first embodiment above. After CMP,terminating approximately along the dashed line 22 shown, the exposedcopper interconnect 12 is passivated using palladium or platinum. The Ptor Pd films 14 may be selectively applied by immersion plating or byelectroless plating. A film ranging in thickness from one monolayer toabout 200 Å may be applied. Following the application of this metalfilm, the device may be annealed at 300-500° C. to interdiffuse theplated metal and copper. Both metals form continuous solid solutionswith copper, and upon annealing, form a hardened surface alloy layerwhich retards the surface transport of copper upon elevated temperatureor EM stress. Both metals are plasma etchable in halide chemistries.Alternately, the heat treatment could be performed later in the process.

Following the surface passivation step insulating films 15, 16, 17, 18,and 19 are deposited as is known in the art. Films 16 and 17 areSiO₂-based or may be one of various low-k insulators as known in theart. These include films listed in the first embodiment. Films 15, 18and 19 are etch stop diffusion barrier layers, such as silicon nitrideor Si—C or PECVD Si—C—N or Si—C—N—O and other “carbide” type films asare known in the art.

As shown in FIG. 4, the stack of insulating films is plasma etched,employing a photoresist layer, not shown, down to etch stop 15, as isknown in the art. After applying another PR layer, not shown, whichdefines the trench region for a second copper based interconnect, notshown completed, the trench region 32 is etched, and film 15 andunreacted or non-interdiffused portions of passivation film 14 areremoved down to the surface of material 12 comprising copper. This maybe seen in FIG. 5.

Following this step, a diffusion barrier 33 is formed as described inthe first embodiment in the form of an edge spacer.

An alternative embodiment is etching this barrier film, which results inedge film 33, to a non-zero thickness of 20 Å or less such that it is apartial barrier in the horizontal regions. In this case, the barrierfilm is electrically conducting.

Following this step, an alloy seed layer 34 is formed as described inthe first embodiment. FIG. 6 shows the device following the formation ofalloy seed layer 34.

It may be noted that the preferred alloying elements Ta and Cr for thealloy seed layer are carefully selected. They require two properties toperform optimally in this invention:

(1) They should be capable of forming strong M-O bonds so that goodadhesion is available against the dielectric barriers such as layer 18.The Gibbs function free energy per oxygen bond for metals decreases as:

Ti>Al>Ta≈Cr>Si>Mo≈W≈Co≈Re>Cu>Ru. The preferred metals also form strongM-N bonds.

(2) They should be immiscible (passive) with copper so that negligibleinterdiffusion occurs from the alloying elements into the main orthicker copper interconnect region formed above and onto film 34.

The main current-carrying copper region above film 34 may be unalloyedor lightly alloyed with elements which only cause small increases inresistivity such as, for example, Ag or Cd. Interdiffusion ofsubstantial concentrations of Ta, Cr, Mo or W would cause significantincreases in the resistivity in the main current-carrying coppermaterial formed onto the alloy seed layers. If this were to happen, thesystem may not be viable. An exemplary alloy is Ta and copper, with theTa concentration approximately 5 to 30 at %. A preferred alloy may be 10to 20 at % Ta in copper. A preferred method of deposition of the seedalloy is PVD or IPVD (ionized PVD) as developed by Applied Materials andothers. The sputtering parameters may be adjusted to maximize edgecoverage over bottom coverage. The films may also be applied by CVD.

Finally, the bulk of the copper based interconnected is formed onto theseed layer and planarized using CMP as described in the firstembodiment. The bulk copper is typically electroplated.

An alternate embodiment for the metal cap layer over copper is nowdisclosed. This barrier, in its more robust form, should both firmlybond to both copper and the overlying oxide insulation or dielectric,and offer passive diffusion barrier characteristics as well. Anexemplary system is the following material stack: platedcopper/displacement plated silver followed by displacementpalladium/selective electroless plated Ru/light oxidation. This systemrequires no overlying dielectric diffusion barrier; thus the ILDdielectric constant is minimized.

FIGS. 7 and 8 show the expected metallurgy of the system upon annealing.A copper interconnect surface such as that of copper region 3 prior toapplication of film 5 is represented in FIG. 7 as electroplated copper35. Approximately 20 Å of immersion silver 36, approximately 10 Å ofimmersion palladium 37, and approximately 100 Å of electroless Ru 38 areapplied over copper layer 35. Upon annealing, at about 400° C. in amixture of N₂ and O₂, solid state diffusion and oxidation of the Ruoccurs producing the structure of FIG. 8 represented by Cu phase 41,Cu+Pd solid solution 42, Ag+Pd solid solution 39, 1-2% Ru in Pd₂ phase43, and conductive RuO₂ surface layer 40. Grain boundaries 44 wouldcontain some oxidized Ru in the remaining Ru layer derived from Ru 38.

Upon application of an overlying dielectric such as film 2, the systemprovides a firm and strongly bound transition from copper to diffusionbarrier to overlying dielectric.

Using non-cyanide complexing agents, various solutions may be preparedfor immersion plating bright films of silver to copper. Silver added tocopper has the smallest known effect on resistivity of all the elements.Since reduction potentials increase as Cu>Ru>Rh>Ag>Pd>Pt>Au, Pd may beimmersion plated over Ag. Such an action also fills any pinholes in theAg layer making the final film more dense.

Solutions for immersion plating of palladium are known. Ru may beelectroless plated using hydrazine as a reducing agent. The final filmstack may be annealed in N₂ followed by N₂/O₂ forming a conductive RuO₂layer and simultaneously stuffing the grain boundaries of the Ru passivebarrier.

For the embodiments where the Ru and RuO₂ layer were at the base of avia, the films could be removed or thinned. Since Ru forms volatilefluorides, this can be accomplished during the anisotropic plasmaetching through the via ILD. This provides a more direct Cu to Cuconnection from via to interconnect.

In recent years, Ru based barriers have been under investigation asdiffusion and oxidation barriers for very high-k, stacked, DRAMcapacitor applications.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed, and that othermodifications and embodiments are intended to be included within thescope of the appended claims.

1. A method for manufacturing an integrated circuit, comprising: forminga copper based interconnect over a substrate using a damascene-typeprocess; depositing a Pd or Pt film over said copper based interconnect;annealing the integrated circuit at a temperature between 300°-500° C.to form a hardened surface alloy layer; forming a dielectric layer oversaid hardened surface alloy layer; forming an opening in said dielectriclayer; removing a portion of said hardened surface alloy layer situatedin said opening; forming a diffusion barrier in said opening; depositingan alloy seed layer in said opening, said alloy seed layer comprisingcopper; and depositing copper in said opening over said alloy seedlayer.
 2. The method of claim 1, further comprising performing aplanarization step using CMP prior to depositing a Pd or Pt film oversaid copper based interconnect.
 3. The method of claim 1, wherein saidPd or Pt film is selectively applied by immersion plating.
 4. The methodof claim 1, wherein said Pd or Pt film is selectively applied byelectroless plating.
 5. The method of claim 1, wherein said Pd or Ptfilm has a thickness between 1 monolayer and 200 Angstroms.
 6. Themethod of claim 1, further comprising forming a diffusion barrier lineron a surface of said interconnect.
 7. The method of claim 6, whereinsaid diffusion baffier liner comprises Ta.
 8. The method of claim 6,wherein said diffusion barrier liner comprises Ti.
 9. The method ofclaim 6, wherein a concentration of Ti in said diffusion baffier lineris between 1% and 30%.
 10. The method of claim 6, wherein said diffusionbaffier liner has a thickness in the range of one monolayer to 200Angstroms.
 11. The method of claim 1, further comprising forming atleast one dielectric diffusion baffier over said dielectric layer. 12.The method of claim 1, wherein said removing a portion of said hardenedsurface alloy layer comprises plasma etching said hardened surface alloylayer.
 13. The method of claim 1, wherein said forming a diffusionbaffier comprises depositing a diffusion baffier in said opening andetching back said diffusion baffier in said opening such that an edgeprotection layer is formed.
 14. The method of claim 1, wherein saidalloy seed layer further comprises at least one of Ta and Cr.
 15. Themethod of claim 1, wherein said copper deposited in said opening oversaid alloy seed layer is a copper alloy.
 16. The method of claim 1,further comprising adding a metal-based cap over said copper depositedin said opening over said alloy seed layer.